MIAMI (Reuters) - The U.S. military judge overseeing the Guantanamo prosecution of five alleged conspirators in the September 11 attacks has issued an order maintaining secrecy over the defendants' experiences in clandestine CIA prisons.

The protective order safeguarding classified information in the case was signed on December 6 by the judge, Army Colonel James Pohl, and unsealed on Wednesday.

It is not limited to documents formally labeled "Top Secret" by the CIA or produced by the government, but also prohibits disclosure of the defendants' own "observations and experiences" in the secret CIA detention, rendition and interrogation program.

Pohl is the chief judge overseeing the war crimes tribunals established by the United States to try foreign captives on terrorism-related charges at the Guantanamo Bay U.S. Naval Base in Cuba.

The defendants in the 9/11 case, including the alleged mastermind of the hijacked plane plot, Khalid Sheikh Mohammed and four other Pakistani, Yemeni and Saudi captives, face charges that could lead to their execution.

All five were held in secret CIA prisons from the time of their capture in 2002 and 2003 until they were sent to Guantanamo in 2006. All have said they were tortured by their U.S. captors and the CIA has acknowledged it subjected Mohammed to the simulated drowning technique of waterboarding 183 times.

Pohl's order prohibits public disclosure of any information that would reveal where the defendants were held; the names, identities and physical descriptions of anyone involved in their capture, transfer, detention or interrogation; and details about the interrogation techniques used on them.

Prosecutors had asked for the order and said it was needed to prevent disclosure of information that could cause grave harm to the security of the United States and its allies.

"By imposing a protective order, he has done what all courts do to responsibly handle national security information while also ensuring both that the accused will receive a fair trial and that the proceedings will be as open and public as possible," a Defense Department spokesman said in response to the ruling.

Defense lawyers and civil liberties advocates called the order an attempt to conceal torture and censor the defendants' own memories. They said the CIA made its interrogation techniques public once it disclosed them to the defendants.

The ruling comes as the Senate Select Committee on Intelligence weighs whether to release a 6,000-plus-page report analyzing the CIA detention and interrogation program that was launched after the 9/11 attacks.

The report is expected to shed light on whether torture produced information that led to the discovery of Osama bin Laden's whereabouts. U.S. special forces killed the al Qaeda leader during a raid at a compound in Abbottabad, Pakistan, in 2011.

Confessions obtained through torture cannot be used as evidence at Guantanamo or in any U.S. court.

Pohl also issued an order maintaining the use of a 40-second delay on the audio feed to the Guantanamo courtroom spectators' gallery.

Journalists, observers and victims' relatives watch the hearings live from behind a soundproof glass wall but the audio is piped in on a delay that gives the court security officer time to muffle the sound if someone mentions information that might be secret.

A consortium of news outlets, including Reuters, argued in October that the delay violated the American public's constitutional right to hear the testimony and failed to meet a requirement that censorship be narrowly tailored.

The American Civil Liberties Union, which also challenged the use of the "censorship chamber," said it would seek further review of Pohl's ruling.

"The government wanted to ensure that the American public would never hear the defendants' accounts of illegal CIA torture, rendition and detention, and the military judge has gone along with that shameful plan," said Hina Shamsi, director of the ACLU's National Security Program.

"The decision undermines the government's claim that the military commission system is transparent and deals a grave blow to its legitimacy."

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